1. Field of the Invention
The present invention relates to a heating unit of an induction heating type, and more particularly to a fixing unit for thermally fixing a toner image on a paper sheet, the toner image having been visualized by toner used as a developer in an electronic photography type copying machine or printer.
2. Description of the Related Art
Japanese Patent Application KOKAI Publication No. 8-76620 has proposed a fixing unit that utilizes induction heating instead of a halogen lamp or heat resistive film. This fixing unit heats a conductive film by magnetic field generation means, and fixes toner on a paper sheet set in tight contact with the conductive film. In the fixing unit, a heat generation belt serving as the conductive film is interposed between a press roller and an assembly serving as the magnetic field generation means to form a nip. Further, Japanese Patent Application KOKAI Publication No. 9-258586 has proposed a fixing unit using a heat generator that has a coil wound around a core disposed along the rotation shaft of a fixing roller and induces eddy currents in the fixing roller to generate a heat.
For the above-described induction heating fixing unit, Japanese Patent Application KOKAI Publication No. 9-120222 discloses a soft-start technique for gradually increasing the power output to a coil up to a predetermined value, in order to suppress a flicker phenomenon that occurs in, for example, an illumination instrument due to power consumption abruptly increased when an RF current begins to flow in the coil. However, the flicker phenomenon depends on a power change per unit time that occurs when the temperature is increased or reduced. Therefore, in the prior art, a sufficiently long setup time is required, which delays the first copying operation. Furthermore, in recent copying machines or the like, for the purpose of power saving, after a predetermined time period elapses after a copying operation, the copy ready state is shifted to a pre-heating state, and then a warm-up state, this state shifting being repeated again and again. However, the power saving effect is reduced if a lot of time is required to increase or reduce the temperature.